25 c.c. of the filtrate are treated in a clean beaker (washed with
dichromate and sulphuric acid) with 12 drops of dilute sulphuric acid
(i: 4) to precipitate the excess of lead and then with 37282 grams of the
powdered dichromate (a] and 25 c.c. of water. When the dichromate is
dissolved, 50 c.c. of the dilute sulphuric acid (g) are added and the beaker
kept in a boiling water-bath for two hours, care being taken to protect it
from organic vapours (alcohol, etc.) and from dust. It is then allowed to
cool, an exactly weighed amount (in excess) of ferrous ammonium sulphate
(e.g., 3-4 grams) being added and the extent of the excess measured by
titration with the dichromate solution (b), potassium ferrocyanide being
used, as before, as indicator.
As it is known from the titration of the ferrous ammonium sulphate
(see c] with how much dichromate i gram of the ferrous salt corresponds,
the quantity of dichromate used in oxidising the glycerine, and from this
the amount of the glycerine, may be calculated : i gram of the dichromate
= 0-13411 gram of glycerine.
As regards these two methods, adopted by the International Commission,
Tortelli and Ceccherelli1 point out that only the second—the dichromate method
—is really exact. The acetin method, according to the accurate investigations
of these authors, gives inconstant and low results.
The same authors also suggest some practical modifications in the dichromate
B. Pure Glycerine
With pure, glycerines the specific gravity is determined and various
common impurities (heavy metals, sulphates, chlorides, oxalates, lime,
arsenic, acrolein, formic acid, fats) and adulterations (sugar, dextrin)
tested for. In some cases the chlorides are determined and possibly the
residue at 160° and other determinations described for crude glycerine.
With dynamite glycerine, a nitration test is made.
1. Specific Gravity.—This is determined by the Westphal balance,
picnometer or hydrometer.
If the glycerine is pure, the content of water may be calculated from
the specific gravity by means of the following table (page 469).
2. Detection of Impurities and Adulterations.—This is effected
by means of the following tests :
(a) i volume of the glycerine is dissolved in 5 vols. of water and the
reaction of the solution tested with litmus paper : pure glycerine should
Aliquot parts of the same solution are then treated with hydrogen
sulphide and with ammonium sulphide to ascertain if heavy metals are
present (blown coloration) ; with barium chloride for the detection of
sulphates, with silver nitrate for that of chlorides, with calcium chloride for
that of oxalates and with ammonium oxalate for that of calcium salts.
(b) i c.c. of the glycerine is treated with 5 c.c. of Bettendorfs reagent
for the detection of arsenic : no coloration should be detectable .within an
1 Annali di chimica appHcaia, 1914, I, p. 514.e of 25 c.c. of the original solution but with 6 c.c. of the